Key Engineering Materials Vol. 591

Abstract: CaO-Al₂O₃-P₂O₅cementitious material (PAC) is a type of new developed hydraulic cement, which has excellent cementitious performance and mechanical property. In cement based materials, the pore structure of hardened cement paste has a significant effect on their performance, such like strength, permeability and durability. In the current paper, pore structure parameters of hardened PAC paste samples in five different curing stages were measured respectively by MIP (Mercury Intrusion Porosimetry). The pore fractal features (pore volume, pore area, porosity) were investigated with fractal theory. Results showed that the porosity of hardened PAC paste does not exactly decline with elongation of curing age and increase of compression strength. The PAC pastes pore structures have typical fractal nature which changes with curing age and pore diameter distribution. The fractal feather of hardened PAC paste can be effectively understood by using porosity fractal dimension.

Abstract: Super light-weight foam concrete with the density less than 250Kg/m³ was prepared by the physical foaming method. The pore structure and microstructure of the super light-weight foam concrete was analyzed by scanning electron microscopy.

Abstract: The effect of WO₃ doping on microstructural and electrical properties of ZnO-Pr₆O₁₁ based varistor materials was investigated. The doped WO₃ plays a role of inhibitor in ZnO grain growth, resulting in decreased average grain size from 2.68 to 1.68 μm with increasing doping level of WO₃ from 0 to 0.5 mol%. When the doping level of WO₃ was lower than 0.05 mol%, the nonlinear current-voltage characteristics of the obtained varistors could be improved significantly with increasing amount of WO₃ doped. But when the doping level of WO₃ became higher, their nonlinear current-voltage performance would be dramatically deteriorated when more WO₃ was doped. The optimum nonlinear coefficient, varistor voltage, and leakage current of the samples were about 13.71, 710 V/mm and 13 μA/cm², respectively, when the doping level of WO₃ was in the range from 0.03 to 0.05 mol%.

Abstract: Alumina ceramics film which has high hardness, wear resistance and erosion resistance can be obtained using micro arc oxidation on the surface of aluminum alloy. The process parameters have great influence on the surface morphology of alumina ceramics film prepared by micro arc oxidation. In this research, the alumina ceramics film was gotten using micro arc oxidation with different process parameters. The surface morphology of ceramics film was analyzed using scanning electron microscope (SEM). The SEM photograph was processed using ImageJ software. The dimensions of holes diameter and porosity on the surface of ceramics film were investigated. The influence regularity of process parameters such as current density, impulse frequency, duty ratio and process time to the surface morphology of alumina ceramics film was researched. This can provide basis for the optimization of process parameters.

Abstract: The modified mullite fiber was used to toughen the mullite ceramic, which can effectively overcome the brittleness of ceramic materials and improve the material strength and toughness. Using 8ZrOCl₂•H₂O as precursor, anhydrous ethanol as solvent, H₂O₂ as hydrolysis promoter and NH₃·H₂O as precipitant, the stable and transparent zirconium sol could be obtained by sol-gel process, which coated mullite short fiber. The results showed that the stable and transparent ZrO₂ sol was obtained when pH value was 3.8~4.8 and reaction temperature was at 25°C. XRD results indicated that monoclinic ZrO₂ particles grew up and formed after heat-treatment at 700°C. The coating ZrO₂ film combined with mullite fiber homogeneously and closely, and the film thickness was about 1 μm.

Abstract: In this paper, (Na_0.52K_0.44Li_0.04)(Nb_0.86Ta_0.06Sb_0.08)O₃ (LTS-KNN) ultrafine-grained ceramic were fabricated over a low and wide temperature range, by using the nanopowder prepared from a water-based sol-gel method. nanopowders are demonstrated to be suitable for preparing fine-grained potassium-sodium niobate ceramics with desirable properties.

Abstract: WC-10Ni cemented carbides with various contents (0-9 wt.%) of ZrC nanopowder were fabricated by spark plasma sintering at 1350 °C with a pressure of 50 MPa. The phase composition, microstructure and mechanical properties of the as-prepared samples were investigated. X-ray diffraction analysis revealed that excepting WC hard phase, a solid solution phase Ni (W,Zr) with different amounts of individual metal was formed with increasing ZrC nanopowder content. Scanning electron microscopy examination indicated that a proper addition content of ZrC can suppress the abnormal growth of WC grains and improve the relative density of WC-Ni cemented carbides. However, with further increase in the content of ZrC (more than 7 wt.%), the agglomeration of ZrC became more and more serious. The samples with 5 wt.% ZrC nanopowder possess a relative higher flexural strength (~1750 MPa) among all the investigated samples. When the addition content of ZrC nanopowder was 3 wt.%, the Vickers hardness of the samples reached its maximum value (~1810 HV₁₀).

Abstract: The solidified TiC-TiB₂ ceramics with excessive Ti mole content were prepared through adjusting the Ti additional agent in combustion system with ultra-high-gravity of 2000 g, and the ceramics were comprised of TiB₂ primary phases, irregular TiC_1-x secondary phases, a few of Al₂O₃ inclusions and Cr-Ti-Al metallic phases. The additional Ti content resulted in not only the rapidly-reduced Al₂O₃ inclusions but also the refined microstructure and the improved homogeneity in the solidified microstructure. As the Ti additional agent was increased, more and more Ti atoms could participate in the formation of nonstoichiometric TiC_1-x phases and Cr-Ti-Al metallic phases. The maximum fracture toughness of 18.5 ± 1.0 MP · m^0.5 in current solidified TiC-TiB₂composite was achieved because of the less Al₂O₃ inclusions, the refined microstructure and the Ti-rich Cr-Ti-Al metallic phases obtained in the near-full-density composite. The high fracture toughness contributed from not only mechanism of crack deflection, crack-bridging and pull-out by a large number of fine TiB₂ platelets, but also the ductile fracture toughening result from plastic deformation of Cr-Ti-Al metallic phases.

Abstract: By taking combustion synthesis in enhanced high gravity field with increasing acceleration from 500 g to 2500 g at intervals of 500 g, a series of TiC-66.7 mol%TiB₂ composites were prepared through liquid forming and rapid solidification. The ceramics were composed of TiB₂ primary phases, TiC secondary phases and a few of Al₂O₃ inclusion and Cr metallic phases. Increasing high gravity acceleration not only promoted the combustion mode transfer from steady SHS mode to explosive one through enhancing the deposition of liquid product toward unreacted blend, but also accelerated liquid-liquid separation of TiC-TiB₂ / Al₂O₃ droplets and subsequent formation of layered melt through enhancing Stokes flow in mixed liquid products, thereby reducing sharply Al₂O₃ inclusions and shrinkage cavities in solidified TiC-TiB₂ composite. Meanwhile, the enhanced Stokes flow brought about constitutional uniformity through enhancing atomic diffusion in TiC-TiB₂ liquid, not only refining the microstructure of the solidified ceramic, but also improving the homogeneity of the ceramic by inducing TiB₂ primary phases to grow at similar velocity. As a result, hardness, flexural strength and fracture toughness of the composite increased greatly in enhanced high-gravity field.

Abstract: Based on taking combustion synthesis in ultrahigh-gravity field to prepare solidified TiC-TiB₂ ceramic, laminated composite of TiC-TiB₂ to Ti-6Al-4V was successfully obtained by fusion bonding, and within the joint there achieved multi-scale (micron-submicron-micro-nano) and multi-level (TiC_1-x-TiB-TiB₂ + TiB₂-Ti-TiC_1-x-TiB + TiB₂-TiC_1-x-TiB-Ti + TiB-TiC_1-x-Ti + TiC_1-x-Ti + Ti) composite structures characterized by size and distribution of TiB₂ and TiB phases. As micro-nano TiB₂, TiB platelets and needle-like nano TiB grains at the joint area initiated residual stress toughening and pinning at crack tip, while Ti phases initiated ductile phase toughening and subsequent bridging in crack-tip wake, a typical delayed fracture under external stress was manifested obviously in the joint of TiC-TiB₂ to Ti-6Al-4V. As a result, the shear fracture of the joint of ceramic toTi-6Al-4V alloy usually occurred in solidified area of Ti alloy rather than the joint, and the shear strength between the ceramic and Ti alloy was maintained at 375 ± 55 MPa.